A raw cylinder head casting is typically finished by machining the deck face and valve seat surfaces to prepare it for installation of spark plugs and valves. Variations in the volume of the combustion chamber have a strong effect on the variation of the compression ratio of the cylinder. Differences in compression ratio between cylinders of the same engine lead to engine noise vibration harshness (NVH). Differences in compression ratio among engines of the same family require using a less than optimum engine calibration for the engine, in order to meet emissions, which reduces power and adversely affects fuel economy. Thus, it is desirable to know and control the volume of a combustion chamber for a finished cylinder head.
The most common method for measuring the volume of a combustion chamber is to first perform the finish machining and then measure the volume of liquid required to fill the chamber. The method involves covering the combustion chamber with a glass plate having a small hole therethrough. The combustion chamber is carefully filled with liquid, which is accurately measured. This method only measures the volume of finished combustion chambers and is laborious and lacks repeatability. Further, this method will not work with an as cast cylinder head, since it requires a totally enclosed volume, i.e., the combustion chamber must contain both valves and spark plug. By being able to predict the finished volume soon after the time of casting, it is possible to adjust the casting process to repeatably produce the desired volume.
One known technique involves predicting the volume of a finished combustion chamber from a raw cylinder head casting as disclosed in "Method of Predicting Volume of Finished Combustion Chambers From a Raw Cylinder Head Casting," filed Jan. 21, 1997, having Ser. No. 08/787,506, now U.S. Pat. No. 5,864,777 the disclosure of which is hereby incorporated in its entirety. In this method, laser scanning is used to create a computer model of the cylinder head geometry. A computer technique called "virtual machining" is then used to emulate the real world machining process and transform the computer model of the raw casting to that of a finished combustion chamber. The volume of the computer model is calculated using numerical techniques.
Gathering data from a raw cylinder head casting utilizing laser scanning involves scanning the combustion chamber surfaces and cast locators, which are used for machining purposes. Laser scanning the cast locators, however, becomes difficult since some of the cast locators are positioned vertically to the deck face of the casting. Since these features are difficult to collect with a non-contact scanner, it is difficult to compute the transformation matrix utilized in the virtual machining process described above.